The U.S. Pat. No. 7,039,063 “Priority Access for Real-Time Traffic in Contention-Based Networks”, refers to a priority access mechanism for stations that need real-time communications, maintaining the possibility of connecting to standard stations. It addresses contention-based networks and claims the usage of a noise sequence (termed black burst) whose duration establishes the priority in the arbitration of the medium access between real-time stations.
This patent applies only to wired communication media, particularly Ethernet, where real-time stations instantly detect the occurrence of collisions. The usage of noise sequences seeks to establish a prioritization of real-time stations with regards to standard stations. At the same time, the use of noise sequences with different durations allows controlling the medium access arbitration among real-time stations.
The patent defines the catenation process of real-time packets transmitted by distinct stations that access the medium in a sequential manner without following the standard guard times for Ethernet. This packet concatenation process guarantees collision avoidance between real-time packets and, since it maintains the medium occupied, it prevents standard stations from initiating transmissions between the real-time packets of the chain.
Nevertheless, given the support of multiple transmission chains (real-time), the occurrence of transmissions from standard stations (between chains) can result in a significant medium access delay for the subsequent chain(s). Such might have a significant impact in real-time transmission delay of subsequent chains. This delay and its variation can became inadequate for a large set of applications such as, for example, the closed loop control (industrial processes, artificial breathing, etc.).
U.S. Pat. No. 6,014,087, “Variable Contention Transmission Media Access Based on Idle Waiting Time”, from the same inventors, establishes an access mechanism for real-time data transmission (video) in IEEE 802.11 wireless networks. The inventors claim a medium access mechanism similar to the one specified in U.S. Pat. No. 7,039,063 without including the notion of transmission concatenation and having the IEEE 802.11 standard as the target technology.
None of the presented solutions, nor its conjunction, allows establishing real-time communications in media where multiple technologies compete for the respective access. This limitation is due to 4 distinct factors:
a) the documents address the existence of communications in a medium and in a spectrum band of a sole technology (for example Ethernet or IEEE 802.11);
b) define black bursts (noise sequences) that are intended to interfere solely with communications of a single technology;
c) do not guarantee medium access timeliness of real-time stations in the presence of other technologies that operate in the same medium over the same frequency band;
d) the solutions presented can not be implemented using commercial transceivers given that they depend on the ability to evaluate the medium without initiating a transmission (a feature inexistent in commercial transceivers).